https://www.youtube.com/watch?v=3aLd8NN0YtY

These aren’t extraterrestrials, they are creatures of the Cambrian Explosion right here on earth, a half billion years ago; the explosion of novel and bizarre life forms that evolution discovered, almost overnight, when it invented multicellular creatures. Dozens of new phyla suddenly appear in the fossil record, apparently fully developed, but only a few survive to this day.

After three billion years of single celled microbes, nature suddenly discovered (or invented!) the metazoa. But these are not “primitive” organisms at all. They are highly sophisticated. Nature can work fast when she wants to, or has to. The jump to multicelluar creatures is what I believe are one of two major bottlenecks in the development of life on earth. The other was the invention of chlorophyll and the rise of photosynthetic plants and oxygen metabolisms. There are profound astrobiological and SETI consequences to that statement.

Although these are all earth creatures, it gives us some idea of workable designs that might easily have arisen on other worlds, without having to speculate about exotic biochemistries like silicon life or sulfur metabolisms.

Look up Stephen Jay Gould’s “Wonderful Life: The Burgess Shale and the nature of history” for more about Cambrian critters, including the remarkable Hallucigenia.

http://en.wikipedia.org/wiki/Hallucigenia